EP0083472A1

EP0083472A1 - Automatic mass spectrometer inlet system

Info

Publication number: EP0083472A1
Application number: EP82306364A
Authority: EP
Inventors: Andrew Barrie; Philip A. Freedman
Original assignee: VG Instruments Group Ltd; V G ISOGAS Ltd
Current assignee: VG Instruments Group Ltd
Priority date: 1981-11-30
Filing date: 1982-11-30
Publication date: 1983-07-13
Also published as: DE3269499D1; US4495414A; DE83472T1; EP0083472B1

Abstract

There is provided a mass spectrometer having a gas inlet system for introducing a sample (1) into the ion source (15) of the spectrometer which inlet system includes a cold trap (11) for condensing a sample (1). The inlet system is provided with means (6) for detecting the pressure therein and means for automatically controlling the operation of the cold trap (11) in dependence on the detected pressure whereby the sample (1) is automatically condensed in the cold trap (11) when it is present in a small quantity. Around the cold trap (11) is conveniently a coolant passage through which coolant from a coolant reservoir is drawn.

Description

This invention relates to an automatic gas sampling inlet system for mass spectrometers, such as those intended for determining the isotopic composition of materials.
One method of determining the isotopic composition of a gas or vapour such as C0₂, S0₂, 0₂, H₂0, etc. is to use a mass spectrometer which is specially constructed for the purpose. These are often small single focussing magnetic sector mass spectrometers which incorporate several fixed collectors, arranged for the simultaneous monitoring of the mass to charge ratios required, for example, 44, 45 and 46 in the case of an instrument intended for C0₂ analysis. In order to admit the sample of gas, a form of gas handling system is required. The simplest of these might consist of a reservoir vessel with an inlet valve which is connected to the spectrometer source by means of a capillary restriction, and a pump for evacuating the vessel when required. A second vessel , containing the sample is connected to the inlet valve, and the reservoir vessel evacuated. The contents of the sample vessel are then expanded into the reservoir vessel, and commence to leak slowly into the spectrometer source through the capillary. The ratios of the intensities of the mass spectrometric peaks corresponding to the mass to charge ratios of interest are then measured in order to determine the isotopic composition of the material. To improve the accuracy, it is conventional to alternate the measurement of the unknown sample with that of a reference sample of accurately known composition, and this is frequently done by using a second, identical inlet system for the reference sample, switching between the two inlets by means of a low volume changeover valve when required.
This valve may also be arranged to connect the sample not in use to a pumping system with the same pressure and pumping speed as the mass spectrometer source pumping system, so that the rate of depletion of both samples is the same, irrespective of which is flowing into the source, thereby avoiding a change in source pressure when the changeover valve is operated.
In order to equalise the pressures in the sample and-reference systems before the measurements are started, a variable volume reservoir (e.g. a stainless steel bellows) is connected to each inlet system.
A mechanism is provided to compress or extend the bellows, either manually or by means of an electric motor and a suitable mechanical linkage, so that the pressure in each inlet system can be adjusted to the desired value after the samples have been admitted. Once the bellows have been adjusted, they are isolated from the rest of the inlet system so that the volumes containing the reference and unknown samples are equal, and the isotopic ratio measurements made as described. By using an inlet system of this type, very accurate results can be obtained, and the entire sample handling routine, including the pressure equalizing technique, can be completely automated if remotely actuated valves are used, controlled by a suitably programmed digital computer. A large number of sample vessels may be connected to a manifold fitted with isolation valves controlled by the computer, so that many samples may be analysed without-the need for operator intervention. Such automatic inlet systems are known, and will not be described in detail.
They suffer, however, from the important defect that a certain minimum quantity of sample is required, generally about 0.1 at.cm³, for their proper operation.
This requirement is due to the minimum internal volume with which an inlet system of this type can be constructed; the sample gas must fill this volume so that the resultant pressure is large enough to ensure an adequate flow of sample into the source. If this flow is too low the mass spectrometer peaks will be less intense, and the accuracy of the measurements will be degraded because of the increased contribution of background noise from the mass spectrometer.
In the case of gaseous samples which can be condensed at atmospheric pressure, such as C0₂, S0₂, H₂0, etc., a better method of handling small quantities of samples is to condense all the available sample into a cooled low volume trap, e.g. 0.1 ccs capacity, to isolate this trap from the sample vessel, pump away any residual non-condensable gases, then connect the trap to the spectrometer inlet restriction, and allow it to warm up. The condensed sample will then vaporise in a much smaller volume than would otherwise be possible, and a higher flow rate into the source can be achieved, at least for a limited period of time. A similar treatment can be applied to the reference sample, and alternate measurements of the isotopic compositions made as previously described. In this way, samples of about 0.01 at.cm³ can be handled, and although the results may not be as accurate as the conventional method used with samples of 0.1 at.cm³ or more, they are considerably better than the results that would be obtained by trying to analyse small samples with the conventional method.
Consequently, there is advantage to be gained by combining a low volume cold trap with the conventional inlet system described, so that the range of acceptable sample sizes can be increased. A difficulty arises, however, in automating an inlet system of this type. It is relatively straightforward, using known techniques, to automate the conventional inlet system, but the construction of a completely automatic cold trap type of inlet system has not been described because of the need to provide at an economical cost, equipment for rapidly cooling and rapidly heating the trap, controlling its temperature, and for automatically detecting which mode of operation of the inlet system is required to suit the sample being analysed without further loss of sample. Known inlet systems therefore comprise a completely automatic conventional inlet system with an additional manually operated cold trap inlet system, which requires the spectrometer operator to heat and cool the trap, e.g. by immersing it in liquid nitrogen, at the appropriate time, as well as operating the various valves throughout the procedure for admitting the sample. It is an object of the present invention to provide an automatic cold trap type of inlet system which can be incorporated in an automatic conventional gas inlet system, and which can be constructed from relatively cheap components, and further, to provide a simple method of detecting which mode of operation of the inlet is required without loss of sample or any operator intervention.
According to one aspect of the invention, there is provided a mass spectrometer having a gas inlet system which includes a cold trap for condensing a sample, characterised in that said inlet system is provided with means for detecting the pressure in said inlet system and means for automatically , controlling the operation of said cold trap in dependence on the detected pressure whereby the sample is automatically condensed in said cold trap when it is present in a small quantity.
The invention enables samples to be automatically analysed when some samples are present in sufficient quantity to be introduced into the mass spectrometer via the conventional inlet system whilst other samples are available in such small quantities that they should be introduced via the cold trap. When a small sample enters the inlet system its pressure will be relatively low and the system is preferably arranged such that when the detected pressure is below a predetermined level on the introduction of the sample the cold trap is automatically operated whilst when it is above the predetermined level the conventional inlet system is used. This may be achieved by providing the inlet system with means for by-passing the cold trap and means for selecting the inlet route by which a sample is admitted to the ion source of the spectrometer to involve either the means for by-passing the cold trap or the cold trap. The means for selecting automatically selects the inlet route to involve the means for by-passing the cold trap when either the detected pressure in the inlet system is greater than a predetermined value or the rate or extent of fall of the pressure in the inlet system whilst the cold trap is maintained at low temperature is greater than a predetermined value.
It sometimes happens that a sample vessel contains a relatively large mass of gas but only a small proportion of it is the relevant sample, the remainder being a non-condensable inert gas. Under these circumstances the cold trap should desirably be used, in effect to concentrate the sample. The invention provides the possibility of automatically detecting these circumstances by detecting the partial pressure of the sample. For example, the cold trap could be , started routinely for all samples and the rate of fall of the pressure monitored. When the gas is largely the condensable sample the pressure will fall relatively rapidly and the conventional inlet system could be used. If the pressure falls relatively slowly, this indicates only a small proportion of sample and the cold trap would then automatically be operated. Thus the invention provides a mass spectrometer inlet system capable of fully unattended operation and capable of selecting automatically the cold trap or the conventional system.
Viewed from another aspect, the invention provides a mass spectrometer having an inlet system including a cold trap, a coolant passage around said cold trap and means for drawing a coolant through said coolant passage from a coolant reservoir. In the preferred embodiment a coolant reservoir is connected to a coolant jacket around the cold trap and an outlet from the jacket is connected to a pump. The flow of coolant, and hence the operation of the cold trap, can be controlled simply by starting and stopping the pump. Alternatively, a control valve, e.g. a solenoid valve, may be installed between the pump and the outlet from the coolant jacket. The pump may then be continuously-running and the flow of coolant controlled by operating the valve. The coolant may be a liquefied gas, such as liquid nitrogen and it may be desirable to provide a heat exchanger between the jacket and the pump (or control valve, if provided) to prevent liquid coolant from reaching the valve and pump. This enables the use of simple and economical components for the valve and pump. The cold trap is preferably also provided with a heating means capable of heating the cold trap to at least 100°C, e.g. an electrical heater, and a temperature measuring means, such as a thermocouple. A known analogue temperature controller may be used to control the heater and solenoid valve in accordance with the sensed temperature to maintain a desired temperature of the cold trap.
Thus a simple and economical cold trap is provided which may be controlled automatically, e.g. by a suitably programmed microprocessor or digital computer. On receipt of a signal from the computer the controller causes the pump to be started, or opens the solenoid valve, so that coolant is drawn through the jacket until the desired temperature is reached. When it is desired to warm the trap to evaporate the sample, the pump is stopped, or the valve closed, and the heater is operated. The pump (or valve) and heater may then be operated to maintain the desired temperature. When it is desired to bake the trap to remove contamination, the heater alone is operated.
The cold trap of the invention is particularly valuable when it is used in combination with an automatic inlet system as defined above since it can of course be brought into operation automatically.
An embodiment of the invention will now be described by way of example and with reference to the accompanying drawings, in which:

Figure 1 shows a mass spectrometer gas inlet system according to the invention;
Figure 2 illustrates the construction of an automatic cold trap of the system of Figure 1; and
Figure 3 shows how the cold trap is connected and controlled; and
Figure 4 illustrates the construction of a further embodiment of a cold trap for the system of Figure 1.

Referring first to Figure 1, it will be seen that the inlet system comprises two identical halves, connected via a changeover valve 14. In the position shown, sample gas flows through restriction 13 into the source 15, whilst reference gas flows through the restrictor 27 into waste pumping system 16. When valve 14 is changed to the "reference" position, the connections are reversed. Pressures in each inlet system can be equalised by variable volume reservoirs 8 and 29, controlled by motors 9 and 30 respectively. These reservoirs are employed only in the conventional mode of operation.
In use, sample vessels 1 with integral manual valves 2 are connected via couplings 3 to isolation valves 4 to manifold 23. The operator attaches the sample vessels, (and reference sample vessels) and evacuates all pipe work up to valve 2 using mechanical pump 18 through valve 20, then high vacuum pump 17 via valve 19. Valves 4 are then closed and the operator opens all the valves 2 on the sample vessels. The rest of the procedure is carried out automatically. Valve 4 on the first sample inlet is opened to expand the contents of the vessel through valve 5 into the small volume bounded by valves 20, 19, lO, 7 and pressure transducer 6. Transducer 6 must be of a low internal volume, be chemically inert, and introduce negligible volume change as it operates. Several commonly available types are suitable, such as those based on strain gauges or the varying inductance of a coil with a core connected to a diaphragm. The digital computer controlling the operation is fed with the signal from transducer 6, and if the pressure is higher than a value previously given to the computer, the mode of operation will be switched to the conventional method, using bellows 8 and motor drive 9 to adjust the pressure in the inlet. This mode need not be described further. If the pressure indicated by transducer 6 is lower than the predetermined value, then the automatic cold trap mode is selected, and the sample is expanded through valve 10 into auto cold trap 11. The volume of trap 11 and valves 10 and 12 is kept to the minimum possible, preferably less than 1 cc. The trap 11 is then automatically cooled in the manner described below, and all the sample contained in vessel 1 is condensed into trap 11. With the form of trap described, this may take between 3 and 5 minutes. The temperature of trap 11 is maintained at the value most suitable for condensing the sample gas, e.g. about -130°C for C0₂ samples.
At this temperature, the vapour pressure of C0₂ is about 0.003 of an atmosphere, which is sufficiently low to avoid significant errors due to the different condensation rates of the different C0₂ isotopes. Any lower temperature will simply increase the time needed to cool the trap without improving the accuracy of the results, whilst a higher temperature may introduce errors, as explained. Other temperatures will be more suitable for different samples. After the appropriate time has elapsed, valve 5 is closed and any residual non-condensable gas is pumped away via valves 19 and 10. Valve lO is then closed, and the trap is heated and maintained at about 20°C (in the case of C0₂) so that the sample becomes gaseous. Valve 12 is then opened to allow the sample to leak through restriction 13 and valve 14 into the mass spectrometer source 15. Whilst the unknown sample is being condensed in trap 11, the reference sample may be condensed in trap 21, and this trap is then heated so that reference gas can flow through restrictor 27 and the other port of valve 14 to waste pumping system 16. Alternatively, the conventional inlet system may be used to admit the reference gas because it is generally available in larger quantities.
The isotopic ratio measurements are then made alternatively on sample gas and reference gas by changing valve 14 until a sufficient number of measurements have been made to ensure the required accuracy. The entire inlet system is then evacuated, first by rough vacuum pump 18 and valve 20, then high vacuum pump 17 and valve 19, using pressure gauges 25 and 26 to ensure that the valves are operated at suitable pressures. Traps 11 and 21 may then be heated to 100°C to remove any contaminating material whilst being pumped by the high vacuum pump, valves 19, and 20 are closed and the trap cooled to room temperature. The analysis of the second sample can then commence.
A possible method of construction of the automatic cold traps is shown in Figure 2. A thick walled tube 33, made of stainless steel, or preferably an inert metallic material which is a good thermal conductor, such as nickel, is attached to the inlet system by flange 32. Its narrow bore extends only about two thirds down the tube, to point 39, and the internal volume should be less than 0.5cc. The upper part of tube 33 is surrounded by jacket 34, through which a suitable refrigerant such as liquid nitrogen can enter through inlet 35 and leave through outlet 36.
The lower part of tube 33 is surrounded by heater 37. The entire trap is surrounded by an insulated jacket 38, and the temperature at the top of the trap is monitored by thermocouple 40.
The method in which the cold trap is operated, and the connection of its auxiliary equipment, is shown in Figure 3. The refrigerant, which is conveniently liquid nitrogen, is stored in vessel 42. It is caused to enter jacket 34 by applying a slight vacuum to pipe 36 from diaphragm pump 46, heat exchanger 44, and solenoid valve 45. A filter 43 protects the system from solid particles which might accumulate in reservoir 42.
When the trap is required to be cooled, valve 45 is opened by controller 41, causing liquid nitrogen to enter jacket 34. When the jacket is full, some nitrogen may enter heat exchanger 44, but the falling trap temperature monitored by thermocouple 40 causes controller 41 to close valve 45 before the exchanger 44 is full, so that no liquefied gas enters valve 45 or pump 46. Exchanger 44 is constructed from copper or another good thermal conductor, so that most of the liquid entering it is vaporized. The temperature in the trap is then controlled by controller 41 opening and closing valve 45 to regulate the flow of liquid gas into the jacket 34 so that the temperature indicated on thermocouple 40 is maintained at a constant value. If the rate of heating of the trap is too low for a satisfactory control action, at the required temperature, heat is applied to the tube 33 by heater 37, which is also controlled by controller 41. By this means the rate of cooling of the trap can be made very rapid, and the final temperature controlled to within +5°C. When it is required to warm the trap to vaporize the condensed sample, valve 45 is closed and heater 37 used to rapidly vaporize any remaining liquid nitrogen, which will be expelled back into reservoir 42 by the expanding gas in jacket 34. Alternatively, an automatically operated air vent valve can be fitted to outlet 47 to admit air so that any expanding gas in line 36 does not bubble back through reservoir 42 causing unnecessary evaporation. The temperature of the trap is then controlled by regulating the power in heater coil 37 in a conventional manner; should the desired temperature be slightly lower than ambient, some refrigerant can be introduced into jacket 34 by opening valve 45 for a short time.
This method of controlling the admission of refrigerant into the cold trap jacket has been found more controllable than the more obvious method of simply pressurising vessel 42, leading to faster cooling times and more stable temperatures than can be achieved by that method.
It will be appreciated that the functions of controller 41, which might consist of conventional analogue electronic circuits, might in many cases be taken over by the digital computer used to control the entire inlet system, or perhaps a satellite computer, based on a microprocessor, and controlled by the main computer, could be used.
Finally, the whole trap can be heated to about 100⁰C by emptying jacket 34 and applying full power to heater 37. This can be used to provide automatic bake out of the trap to remove any contaminating materials before the next sample is introduced.
Another form of cold trap suitable for use in the invention, which is especially suitable for use with liquefied gas coolants, is shown in Figure 4. It consists of a thin walled tube 47, typically made from stainless steel, which is attached to the inlet system by flange 48. Tube 49 is closed off by diaphragm 49, and is surrounded by an inner vessel 50, which is open at the top, as shown. A thermocouple 51 is inserted into the lower part of tube 47, which has a narrower bore than the top section, so that its hot junction is adjacent to diaphragm 49. Liquid coolant enters the bottom of inner vessel 50 via pipe 52, and cools the tube 47. Evaporating coolant, which is a gas at low temperature, escapes from vessel 50 and fills outer vessel 53. Inlet pipe 52 is concentrically surrounded by another pipe 54 which is connected to the lower part of outer vessel 53. Pipe 54 serves as an outlet for the coolant and is connected to pump 46 (Figure 3) via a valve 45, if desired. The outer wall of vessel 53 and pipe 54 is wound with an electrical heating element 55 which is usually energized at low power even when coolant is flowing through the trap. This results in vaporization of any liquid coolant which might enter the outer vessel 53 from inner vessel 50, and ensures that only gaseous coolant leaves the outlet pipe 54. Heat exchanger 44 (Figure 3) between the trap outlet and pump 46 is therefore not required with this embodiment and can be omitted. In addition, the cold gas surrounding inner vessel 50 serves as a thermal insulator, and prevents excessive loss of coolant by premature evaporation, and because the temperature of the wall of the outer vessel 53 is maintained above the surrounding temperature by heater 55, even when coolant is flowing, the condensation of water from the atmosphere is eliminated. In other respects, the operation of this type of trap is similar to the embodiment described previously, full power being applied to the heater when it is desired to bake the trap or vaporize the sample rapidly.

Claims

1. A mass spectrometer having a gas inlet system which includes a cold trap for condensing a sample, characterised in that said inlet system is provided with means for detecting the pressure in said inlet system and means for automatically controlling the operation of said cold trap in dependence on the detected pressure, whereby the sample is automatically condensed in said cold trap when it is present in a small quantity.

2. A mass spectrometer as claimed in claim 1 wherein said means for automatically controlling the operation of said cold trap operate to cause condensation of the sample in said cold trap if said detected pressure is lower than a predetermined value.

3. A mass spectrometer as claimed in claim 1 wherein said inlet system further includes means for detecting the rate of fall, or the extent of fall, of the pressure in the inlet system whilst the said cold trap is maintained at low temperature, and means for comparing the said rate of fall, or extent of fall, with a predetermined value, whereby if the said rate of fall or extent of fall is less than said predetermined value, said means for automatically controlling the operation of said cold trap operates to cause condensation of the sample in said cold trap.

4. A mass spectrometer as claimed in claim 2 or 3 further including means for removing noncondensable components of the sample after the condensable components have been collected in said cold trap.

5. A mass spectrometer as claimed in any one of claims 1 to 4 further including means for automatically isolating from said inlet system a sample vessel from which a sample may be introduced into said inlet system, following introduction of said sample into said inlet system and condensation of said sample in said cold trap, and means for vaporising the sample condensed in said cold trap into a volume substantially smaller than that of said sample vessel.

6. A mass spectrometer as claimed in any one of claims 1 to 5 wherein said inlet system further includes means for by-passing said cold trap and means for selecting the inlet route by which a sample is admitted into the ion source of the mass spectrometer to involve either said means for by-passing or said cold trap, said means for selecting being capable of automatically selecting said inlet route to involve said means for by-passing when either the detected pressure in said inlet system is greater than a predetermined value or the rate or extent of fall of the pressure in said inlet system while said cold trap is maintained at low temperature is greater than a predetermined value.

7. A mass spectrometer as claimed in any one of claims 1 to 6 wherein said means for automatically controlling the operation of said cold trap, which may optionally be capable of controlling other operations involved in the admission of a sample into said inlet system and into the ion source of the mass spectrometer, comprises a suitably programmed microprocessor or digital computer.

8. A mass spectrometer having an inlet system including a cold trap, a coolant passage around said cold trap and means for drawing a coolant through said coolant passage from a coolant reservoir.

9. A mass spectrometer as claimed in any one of claims 1 to 7 wherein said inlet system further includes a coolant passage around said cold trap and means for drawing a coolant through said coolant passage from a coolant reservoir.

10. A mass spectrometer as claimed in either of claims 8 and 9 wherein said coolant reservoir is connected to said coolant passage and an outlet from said coolant passage is connected to a pumping means through a control valve capable of being operated by an automatic control means.

11. A mass spectrometer as claimed in either of claims 8 and 9 wherein said coolant reservoir is connected to said coolant passage and an outlet from said coolant passage is connected to a pumping means capable of being operated by an automatic control means.

12. A mass spectrometer as claimed in claim 10 or 11 wherein said coolant reservoir and coolant passage are adapted for use with a liquefied gas as said coolant and wherein a heat exchanging means is provided between said coolant passage and said pumping means or said control valve if provided and is so constructed as to ensure that said coolant is vaporised before entering said control valve or said pumping means.

13. A mass spectrometer as claimed in claim 10 or 11 wherein a vent valve is provided at the connection between said coolant passage and said pumping means or said control valve if provided and is capable of automatically admitting air into said coolant passage thereby to allow a rapid return of said coolant into said coolant reservoir.

14. A mass spectrometer as claimed in claim 10, 11 or 13 for use with a coolant consisting of a liquefied gas, in which the said coolant passage includes an inner vessel surrounding the cold trap and open at its upper end and substantially enclosed by an outer vessel from which the coolant is withdrawn after passing through the coolant passage.

15. A mass spectrometer as claimed in claim 14 in which the said outer vessel is provided with a heating means capable of ensuring the vaporisation of any liquid coolant entering the outer vessel.

16. A mass spectrometer as claimed in claim 15 in which the said heating means is additionally capable of heating the said cold trap to at least 100°C in the absence of any coolant passing through the trap.

17. A mass spectrometer as claimed in any one of claims 8 to 13 wherein said cold trap is provided with a heating means capable of heating said cold trap to at least 100°C.

18. A mass spectrometer as claimed in any of claims 8 or 10 to 13 wherein said cold trap is provided with a temperature measuring means, and wherein an automatic control means is provided which is capable of controlling the operation of whichever of the said control valve, pumping means, vent valve, and heating means as are also provided to maintain the temperature of said cold trap at a desired value.

19. A mass spectrometer as claimed in any one of claims 1 to 14 having two of said gas inlet systems connected to the ion source of the mass spectrometer through a changeover valve capable of automatic operation, wherein one said gas inlet system is capable of being used to admit a sample of a known composition into said ion source for use as a reference and the other said gas inlet system is capable of being used to admit a sample for investigation into said ion source.